this is just disgusting!
I agree that the right click disable is worthless as everyone knows how to use the print screen.
Definitely use a watermark and make all online images: max of 72dpi.
With regards to COA are you a member of ebsq?
it is a paid membership (well worth the price)
you can upload art and create COA that are then activated/locked by the buyer using a code supplied on the COA...if the buyer wants to sell the artwork, they need to contact you so you can then 'unlock' the COA.
It's a good idea, that way you know where your art is.
Send out a press release/announcement about the copyright infringements.
Educate your buyers!
Goodluck, i really hope this gets resolved.